This invention relates generally to the art of blow molding and more particularly to the art of blow molding utilizing large parisons.
The art of blow molding has traditionally been carried out by vertically extruding a tubular form in a downward direction followed by subsequent steps of enclosing the tubular form within a mold section located immediately adjacent the extruder outlet and inflating the thus entrapped parison to form an article in substantial conformity with the mold geometry.
A problem with vertical extrusion is that as the tubular form is extruded, the weight of the extruded material begins to bear upon that material which supports it. This effect is most pronounced at the extruder orifice which bears the entire weight of the extruded material. Thus, the longer the tubular form, the greater the weight bearing upon the supporting extruded material and the greater the stretch or sag produced in the material bearing the weight. This sag tends to produce a parison which does not have a uniform cross section.
In the past, when parison weight has been sufficient to cause excessive sag, hot thermoplastic material within the extruder has been collected within an accumulator while the parison is in the process of being molded. The accumulator prevents further emission of any thermoplastic material from the die while the parison is in the forming or blow molding stage of the process. The accumulator, usually in the form of a piston and cylinder, serves the additional purpose of quickly extruding the thermoplastic material through the die orifice in order to minimize the time during which sagging can occur.
The degree of sagging which is produced during any process is to a large extent a function of the hot strength of the thermoplastic material being formed into a parison. The most desirable material is high molecular weight polyethylene because of its hot strength. Other materials, however, possess desirable attributes but have hot strength such that the utility of such materials within a blow molding process is severely limited. Materials which possess only marginal hot strength as far as a blow molding process is concerned include but are not limited to nylon, polypropylene, saran, acrylonitrile-butadiene-styrene copolymers, polycarbonates and polystyrene. These materials all present significant sagging problems.
A technique to overcome the problem of non-uniformity produced by the sagging of a tubular form is described in U.S. Pat. No. 3,759,719 to Morecroft et al. The process described therein comprises the production of a tubular form having varying cross-sectional width at the extruder outlet. The process described by Morecroft et al comprises the continual monitoring of the tubular wall thickness and constant compensation for any irregularities therein as compared to a previous setting. Such programming may be utilized so as to minimize the effects of sag produced by the weight of the extruded tubular form.
An additional problem with conventional blow molding processes is the difficulty of extruding tubular forms from materials which are thermally degradable. Such materials tend to degrade or decompose as a function of time at temperature. Normally such materials are desirably extruded from extremely streamline dies so as to minimize the residence time within the heated die. Accumulators by their very nature are nonstreamline. In attempting to accumulate thermally degradable materials, the long stay as well as the stagnant areas within the nonstreamlined construction and severe degradation make it virtually impossible to produce quality products. For this reason, only materials which are not thermally degradable have been heretofore utilized in blow molding processes utilizing accumulators.
The prior art has devised processes for producing coextruded blow molding objects. A coextrusion die useful in a blow molding process is described in U.S. Pat. No. 4,149,839 to Iwawaki et al. The vertical coextrusion process described therein utilizes a series of mandrels and piston rings surrounding rods associated with the mandrel which function to both extrude and accumulate materials within the various layers. Thermal degradation within a coextrusion die becomes a particular problem when one of the materials must of necessity be at a significantly higher temperature than the degradable material. This problem is particularly acute near the point of conversion of the two layers where the degradable material tends to remain motionless during the accumulating process.